1. Field of the Invention
The present invention generally relates to a line arrester for use in a support mechanism for a power transmission line (hereinafter referred to as "power line"). More particularly, this invention pertains to a line arrester which can promptly ground the surge current generated by lightning striking in the power line, and cut off the follow current to prevent ground failure.
2. Description of the Related Art
FIG. 1 shows a typical line arrester which supports a power line 50 in an insulated manner and absorbs any lightning surge currents generated by a lightning strike in the power line 50. This line arrester includes a metal upper hanger 52, a line arresting insulator string 53 and a metal lower hanger 54 by which the power line 50 is suspended from a tower arm 51. The line arresting insulator string 53 is constructed by linking multiple line arresting insulators 55 in series. The insulators 55 cope with the lightning surge current.
As shown in FIG. 2, a body (porcelain shell) 56 of each line arresting insulator 55 has a shed 55a with a pair of bore holes 56c, and a head 56b integrally formed on the center top portion of the shed 56a. A metal cap 57 is fixed to the top portion of the insulator head 56b, and a metal ball pin 58 is secured to the bottom portion of the head 56b.
A plurality of variable resistors (hereinafter referred to a "varistors") 59 are accommodated in each bore hole 56c. Each varistor 59 consists essentially of zinc oxide and has a non-linear voltage-current characteristic. The varistors 59 are retained in each bore hole 56c by an upper seal 60 and a lower seal 61, respectively attached to the upper and lower end sections of that bore hole 56c.
The upper seal 60 is connected via a bonding wire 62 to the cap 57, while the lower seal 61 is connected via a bonding wire 63 to the pin 58. The cap 57 is provided with arc guides 64 in association with the upper seals 60. The line arresting insulators 55 are arranged one above another and are coupled together by the engagement of the pin 58 of an upper arresting insulator with the cap 57 of a lower arresting insulator.
In this line arrester, the upper hanger 52 and lower hanger 54 are respectively provided with arcing horns 65 and 66 as shown in FIG. 1. The length of the air gap between the upper and lower arcing horns 65 and 66 is determined so as not to cause flashover between the arcing horns even in the case where a critical discharge current flow through each arresting insulator 55.
When the lightning surge current generated by a lightning strike in the power line 50 is at an expected normal level, the lightning surge current is discharged in the ground, passing through the lower hanger 54, the line arresting insulator string 53, the upper hanger 52 and the arm 51. At this time the lightning surge current passes the pin 58, wire 63, varistors 57, wire 62 and cap 57 of each arresting insulator 55 in the line arresting insulator string 53. After discharging the lightning surge current, the varistors 57 suppress or cut off the follow current to thereby prevent ground faults of the power line.
When the lightning surge current generated in the power line 50 is so large as to exceed the critical discharge current of the varistors 59, this lightning surge current will unavoidably break the varistors 59. The destruction of the varistors 59 causes an arc generated by the follow current to run through the bore hole 56c. This arc induced by follow-current is directed outward by the arc guides 64 and is promptly led to a region between both arcing horns 65 and 66.
The conventional line arrester, however, is designed on the assumption that the varistors 59 will inevitably be broken by an excessive lightning surge current which is greater than the design value. To recover the permanently grounded state and supply electricity, therefore, it is necessary to replace all the broken arresting insulators with proper ones. Since the replacement of the insulators takes time, it is difficult to quickly restore the power transmission system. In addition, this job increases the repairing cost required at the restoring time.
When the varistors 59 are broken as mentioned above, arc indused by the follow-current should move such that it runs between the arcing horns 65 and 66 through the arc guides 64. However, the distances between the individual arcing horns 65 and 66 and their associated arc guides 64 are set very large in the conventional line arresting insulator, making it difficult to lead the arc towards the arcing horns 65 and 66. It is noted that if the arc generated by the follow-current continues running along the outer surface of the insulator string, it burns out the line arresting insulator string 53. In the worst case, the arresting insulator string 53 may be cut off at some point. In such a case, the line arrester can no longer support the power line 50.